permaculture

["robscott@freeshell.org" <robscott@freeshell.org>]

In response to the disparaging comments about Black Walnut, I'm posting my 
research on Black Walnut ecology and companion planting. A list of 
productive species for Black Walnut understory appears at the bottom.

Rob Scott
Urbana Permaculture Project
Urbana, IL

Black Walnuts (Juglans nigra) and Hackberry (Celtis occidentalis) occur 
together naturally, both are reported to be allelopathic, for separate but 
overlapping sets of plants. In warmer climates, the closely related 
Sugarberry (Celtis laevigata) would be a more fruitful associate.

Why is Black Walnut toxic?

Black Walnuts produce a toxin called "juglone" in all parts of the tree. 
It is an antirespiratory compound with a phenolic ring. It probably 
evolved to deter insect pests, not plants or humans. Fishermen have used 
ground up walnut hulls to attract fish to the surface of the water; the 
juglone slows fish respiration and the fish come to the surface to 
breathe. Juglone in leaf litter degrades in about 2 days, so raked up leaf 
mulch is not toxic. Walnut tree roots constantly release juglone, and this 
is why nearby plants often die.

What companion plants help Black Walnut grow?

Control experiments in Illinois found that Autumn Olive (Elaeagnus sp.) 
work better as a symbiont for promoting Black Walnut growth than Black 
Locust (Robinia pseudoacacia). Both fix nitrogen, but the Autumn Olive 
doesn't compete with the Black Walnut as much and seems to reduce deer 
browse. The net effect of Black Locust was to set back Black Walnut by 
shading. Degraded sites will yield different results as Black Locust 
better restores depleted soils.

What naturally grows under Black Walnut?

Redbud (Cercis canadensis), Pawpaw (Asimina triloba), and Spicebush 
(Calycanthus occidentalis) grow as understory shrubs under Black Walnut 
naturally in Central Illinois. Ground level  wildflowers are no less 
abundant under Black Walnuts than in other natural forest communities.

Does Black Walnut create a lot of shade?

Black Walnuts let through a lot of sunlight. Shade is "dappled" even after 
leaves have fully emerged. Bud break in Black Walnut occurs in late 
spring, allowing sun penetration to the forest floor for a few months 
before leafing out. A rust fungus commonly causes anthracnose in Black 
Walnut leading to early leaf-drop and dormancy in the fall, adding another 
month of full sun penetration. Understory shrubs have been shown to reduce 
anthracnose infection by breaking the ground-to-leaf cycle.

What productive plants can grow under Black Walnut?

(in no particular order)
American Black Currant - Ribes americanum
European Currant - Ribes nigra, Ribes rubrum, Ribes alba
Missouri Gooseberry - Ribes missouriensis
American Elderberry - Sambucus canadensis
Black Raspberry - Rubus ideaus
Siberian Pea Shrub - Caragana arboescens
Mulberries - Morus nigra, Morus alba
Alpine Strawberries - Fragaria vesca
Goldenseal - Hydrastis canadensis
Wild Ginger - Asarum canadense
Pawpaw - Asimina triloba
Redbud - Cercis canadensis - legume
Bamboo - Phyllostachys spp.
"Rose of Sharon" Hibiscus - Hibiscus Syriacus
Lamb's Quarters - Chenopodium alba
Catnip - Nepata cataria
Daylily - Hemerocallis sp
Sunchokes - Helianthus tuberosa

This is a good way to control the spread of sunchokes, and the tubers are 
of fine size and quality even when shade grown. Dandelions, Violets and 
Creeping Charlie have also grown naturally in Illinois Black Walnut 
Guilds. Toby Hemenway mentions Woldberry (Lycium spp) in Gaia's Garden for 
growing near Walnut, but it doesn't grow well in Illinois.